Optical fibers have become increasingly important as a medium for transmitting large quantities of information in the form of lightwaves. Each optical fiber comprises a thin solid glass cylinder, known as the core of the fiber, surrounded by a glass sleeve, known as the cladding, which has a lower refractive index than the core. There are a number of ways of making optical fiber, one way being, first, to construct a glass tube, and then to vapor-deposit glass that will eventually constitute part of the cladding and the core on the inner surface of the glass tube by the method taught, for example, in the U.S. patent of MacChesney, et al. No. 4,217,027 granted Aug. 12, 1980, now generally known as the modified chemical vapor deposition (MCVD) technique. When used in such manner, the glass tube is known conventionally as a substrate tube. After deposition, the entire structure is collapsed to make a preform rod, and glass drawing techniques are used to form a continuous optical fiber from the preform. The vapor deposited glass of the optical fiber may be doped so as to have a refractive index that is higher and/or lower than that of undoped glass; in any event, the refractive index of the core must be higher than that of the cladding layer for efficient transmission of lightwaves in the usual manner. One way of obtaining this difference is to dope the cladding layer with fluorine, which depresses the refractive index with respect to that of undoped glass (silicon dioxide).
For large preforms, an oversized core is deposited in the substrate tube and the collapsed preform rod is inserted into a second tube, called an overclad tube, that is collapsed onto the rod. This is the hybrid or "rod-in-tube" approach as described, for example, in the U.S. patent of J. W. Baumgart et al., No. 4,820,322, granted Apr. 11, 1989. One may likewise wish to dope such overclad tubes with fluorine to depress their refractive indices.
Both substrate and overclad glass tubes may be made, first, by depositing glass soot on a mandrel to form a cylindrical porous soot form of glass particulate. The deposited porous soot cylinder is then consolidated into a glass tube by heating the soot cylinder in a furnace. Prior to consolidation, fluorine can be introduced as a dopant by adding a fluorine containing gas to the reactant stream of the torch during soot deposition, or by adding a fluorine containing gas to the furnace atmosphere at a temperature below the consolidation temperature. During the consolidation step, preferential volatilization of fluorine from the porous cylindrical tube surfaces typically produces a non-uniform distribution of fluorine through the thickness of the consolidated glass tube. Such non-uniformities can cause problems with the efficiency of lightwave transmission and reproducibility of the fiber.
One approach to this problem is to consolidate glass soot previously doped with fluorine while flowing a fluorine-containing gas over it in the furnace. Another proposal, described in the U.S. patent of Berkey, No. 4,629,485, granted Dec. 16, 1986, is to dope the previously undoped soot during the consolidation step by flowing fluorine gas about the heated soot cylinder before and during consolidation. We have found that it is difficult with either of these approaches to obtain the desired uniformity of fluorine doping through the thickness of the final tube; also, such processes increase the expense of production because the highly reactive fluorine, at the high temperature required for consolidation, inevitably damages various furnace parts during its use.
Accordingly, there is a well-recognized need in the industry for a convenient and inexpensive method for doping a glass tube, destined to be the cladding layer of an optical fiber, with a refractive index reducing, and highly reactive, dopant such as fluorine, in such manner that the reduced refractive index is substantially uniform through the thickness of the tube. There is also a need for such a method that does little or no damage to furnace parts.